1. Field of the Invention
The invention relates to an optical communication system for transmitting subscriber-assigned information signals in two directions between a center and a plurality of subscribers.
2. Description of the Prior Art
A communication system of this type is known from EP-A2-0 164 652. A distribution of information signals, e.g., television signals, from the center to the subscribers is not provided in this. Only bidirectional information services, also called interactive services, are to be handled by the system.
It would be desirable to have a solution in which not only bidirectional but also unidirectional services, i.e., distribution services, can be handled. For this, the following solutions are known:
a) EP-B1-0 071 232 provides the teaching of using two optical waveguides for each subscriber: One for television and radio transmission, i.e., for the distribution services, and the other for the bidirectional information services. This does not involve an integrated system, but two systems independent of each other, and there is the disadvantage that large lengths of optical waveguides are required altogether.
b) From "IEEE Journal on Selected Areas in Communications", Vol. SAC-4, No. 4, July 1986, pp. 551-564, it is known (p. 554) to expand a system of the type mentioned above (FIG. 5 on p. 554) into a service-integrated system for interactive and distribution services by the fact that the signals for the transmission of television and radio programs are added by time-division multiplexing to the signals to be transmitted by the center to the subscribers for the interactive services, and that the bit sequence frequency for this transmission direction is thereby increased. This type of integration appears meaningful from economic viewpoints, but may be undesirable for political or legal reasons if the distribution services and the interactive services are assigned to different operators.
A purely optical distribution system which, like the interactive system mentioned at the beginning, contains a passive optical remote distribution unit is known from DE-OS 32 20 817.
Aside from the above-mentioned integration of the distribution services into a system for handling the interactive services by time-division multiplexing and by increasing the bit sequence frequency, no proposal is known for how an optical transmission system can be created for the subscriber area with which both the distribution services and the interactive services can be handled.